The invention relates to a deflection unit for a colour cathode-ray tube, which unit is also called a deflection yoke, which includes a pair of horizontal deflection coils and a pair of vertical deflection coils in the form of a saddle, the particular shape of which makes it possible to minimize the coma, geometry and convergence errors of the beams simultaneously.
A cathode-ray tube intended to generate colour images generally comprises an electron gun which emits three coplanar electron beams, each beam being intended to excite a phosphor for a specific primary colour (red, green or blue) on the screen of the tube.
The electron beams scan the screen of the tube due to the effect of the deflection fields created by the horizontal and vertical deflection coils of the deflection yoke fixed to the neck of the tube. A separator made of plastic serves to isolate the two pairs of coils and to ensure that the deflection yoke is mechanically rigid. A ring of ferromagnetic material conventionally surrounds the deflection coils so as to concentrate the deflection fields in the appropriate region.
The three beams generated by the electron gun must always converge on the screen of the tube on pain of introducing a so-called convergence error which falsifies, in particular, the rendition of the colours. In order to make the three coplanar beams converge, it is known to use so-called self-converging astigmatic deflection fields; in a self-converging deflection coil, the intensity of the field or the lines of flux which are caused by the horizontal deflection winding are generally in the form of a pin-cushion in a portion of the coil which lies more to the front of the latter on the side of the tube which faces the screen. This amounts to introducing, into the distribution of the turns making up the line coil, a highly positive 3rd harmonic of the ampere-turns density at the front of the coil.
Moreover, due to the action of uniform horizontal and vertical deflection magnetic fields, the volume scanned by the electron beams is a pyramid, the apex of which is coincident with the centre of deflection of the deflection yoke and the intersection of which with a non-spherical screen surface exhibits a geometrical defect called pin-cushion distortion. This geometrical distortion of the image is all the greater the larger the radius of curvature of the screen of the tube. Self-converging deflection yokes generate astigmatic deflection fields making it possible to modify the North/South and East/West geometry of the image and, in particular, partially compensate for the North/South pin-cushion distortion.
The design of the deflection yoke must also take into account the coma, which is an aberration affecting the lateral beams emanating from an electron gun emitting three beams in line, independently of the astigmatism of the deflection fields and of the curvature of the screen surface of the tube; these lateral beams enter the deflection zone at a low angle with respect to the axis of the tube and undergo a deflection in addition to that of the axial beam. The coma is generally corrected by modifying the distribution of the deflection fields at the point where the beams enter the deflection yoke so that the coma generated compensates for that produced by the distribution of the field which is necessary in order to obtain the desired astigmatism for achieving self-convergence. Thus, with regard to the horizontal deflection field, the field at the rear of the deflection yoke is in the form of a barrel and in the front part in the form of a pin-cushion.
In addition, the two, horizontal and vertical, pairs of deflection coils must generate deflection fields which are strictly perpendicular to each other. If the two fields are not perpendicular, a phenomenon occurs in which one field is modulated with respect to the other. Since the amplitude of the horizontal deflection coil control signals is about 900 volts whereas the vertical deflection coils are supplied with approximately 50 volts, the vertical deflection coils act as the secondary of a transformer, the primary of which would be the horizontal deflection coils. This modulation effect, also called cross-modulation, is more commonly defined by the expression X Mod=100×Vv/Vh, where Vv is the voltage measured at the vertical deflection coils when the horizontal deflection coils are supplied with Vh. The cross-modulation generates geometrical problems in the image created on the screen of the tube by the scanning of the electron beams.
These problems are, for example, orthogonality and parallelogram faults. The correction of these faults requires them to be taken into account when designing the deflection yoke, but this process of taking them into account is difficult, or even impossible, since the defects result from manufacturing problems which arise in the first phases of manufacturing the deflection yoke, when the design stage has been completed; hitherto, it was therefore necessary to remedy these problems either by introducing a new step in the design of the deflection coils or by using electronic control circuits capable of being responsible for the geometrical corrections of the image. The invention provides a simple solution to these problems, by modifying the front conductor assembly of a pair of saddle shaped coils and by introducing a shape asymmetry into said conductor assembly. This modification is introduced into the manufacture by modifying the shape of the coil shaper placed in front of the mould in which the winding takes place, which modification has no effect on the other parameters defined in the design of the coil, such as the convergence of the three electron beams or the coma.